There has been a practice of estimating a move direction of a moving body on the world coordinate system on the basis of measurement data generated by a self-contained sensing device (for example, an acceleration sensor, a magnetic sensor, a gyroscope, or an atmospheric pressure sensor) built in a terminal held by the moving body (for example, a walker) (Non Patent Literature 1).
Estimating a move direction of a moving body on the world coordinate system requires estimating the following three vectors simultaneously on a sensor coordinate system of the self-contained sensing device: (1) a gravitational direction vector, (2) a horizontal reference direction vector (for example, the direction of true north), and (3) a travel direction vector.
Estimating (tracking) the above vectors (1) to (3) allows estimation of the angle formed by the travel direction with respect to the horizontal reference direction, thereby allowing estimation of the azimuth of a move of the walker.
On the basis of triaxial data outputted separately by an acceleration sensor of the gravitational direction vector (1) and an angular velocity sensor, tracking a gravitational direction on the sensor coordinate system can be estimated.
The horizontal reference direction vector (2) can be estimated by tracking a horizontal reference direction on the basis of data outputted by an acceleration sensor, an angular velocity sensor, and a magnetic sensor.
Non Patent Literature 1, for example, specifically discloses a technique of tracking a gravitational orientation vector and a horizontal reference direction vector. There has also been known a technique based on a so-called attitude and heading reference system (AHRS).
For the travel direction vector (3), technical issues to be considered greatly differ between (i) the case in which the self-contained sensing device is fixed to, for example, a foot or lower back of a walker as a moving body and (ii) the case in which the self-contained sensing device is built in a terminal and the posture in which the walker holds the terminal changes freely.
For instance, in the case where the self-contained sensing device is fixed, it has a fixed positional relationship with the walker. This makes the travel direction of the moving body generally already known, thereby makes it relatively easy to track a travel direction. On the other hand, in the case where the self-contained sensing device is held at a position and in a posture that freely change, it is not easy to track a travel direction.
In view of the above, there have been proposed various methods for estimating a travel direction (Patent Literatures 1 and 2). Those methods have, however, failed to estimate a travel direction accurately.
Non Patent Literature 2 discloses that a person's walk motion has an acceleration component and an angular velocity component that have frequency characteristic patterns as shown in Table 1 below as the acceleration vector (triaxial) and angular velocity vector (triaxial) are correctly divided into (i) a component in a travel direction, (ii) a component in a vertical direction (gravitational direction), and (iii) a component in a side-to-side direction (lateral direction) orthogonal to the travel direction and vertical direction.
TABLE 1AccelerationAngular velocitycomponentcomponentVertical directionWalk frequencyWalk frequency ×axis (yaw axis)½Side-to-sideWalk frequency ×Walk frequencydirection axis½(pitch axis)Travel directionWalk frequencyWalk frequency ×axis (roll axis)½
In other words, in the case where an acceleration vector and an angular velocity vector can each be divided into different components in such a manner as to satisfy the characteristics shown in Table 1, those directions can be regarded as an optimal travel direction and an optimal side-to-side direction (lateral direction).
Non Patent Literature 2, however, merely suggests, as an algorithm for estimating a travel direction, continuously changing the azimuth by a necessary resolving power. Non Patent Literature 2 thus fails to disclose any specific method corresponding to such a suggestion.
Patent Literature 3 discloses, for example, a device and the like for comparing (i) characteristic patterns of acceleration, angular velocity, and the like arising from a typical walk motion of a human being's in such directions as the travel direction with (ii) actual measurement data to estimate a travel direction of the walk motion. Patent Literature 4 discloses a technique that, to identify a person's action, utilizes a cross-correlation between acceleration and angular velocity or a phase difference in a frequency domain.